Use the FILE_SHARE_DELETE flag for CreateFile on a mapping

A comment by rransom on #8795 taken together with a comment by doorss
recorded on #2077 suggest that *every* attempt to replace the md cache
will fail on Vista/Win7 if we don't have the FILE_SHARE_DELETE flag
passed to CreateFile, and if we try to replace the file ourselves
before unmapping it.  I'm adding the FILE_SHARE_DELETE, since that's
this simplest fix.  Broken indexers (the favored #2077 hypothesis)
could still cause trouble here, but at least this patch should make us
stop stepping on our own feet.

Likely fix for #2077 and its numerous duplicates. Bugfix on
0.2.2.6-alpha, which first had a microdescriptor cache that would get
replaced before remapping it.